1. Technical Field
The present invention relates to a separator for polymer electrolyte fuel cells and to a method of manufacturing therefor.
2. Background Art
A unit of a polymer electrolyte fuel cell is formed by laminating separators at both sides of a tabular membrane electrode assembly (MEA), and then plural units are laminated to form a fuel cell stack. The membrane electrode assembly is a three-layered structure in which an electrolyte membrane composed of an ion exchange resin or the like is arranged between a pair of gas diffusion electrodes forming a cathode and an anode. The gas diffusion electrode is a structure in which a gas diffusion layer is formed on an outer surface of an electrode catalytic layer contacting the electrolyte membrane. The separator is laminated to contact the gas diffusion layer of the membrane electrode assembly, and a gas passage in which gas flows and a refrigerant passage are formed between the gas diffusion electrode and the separator. In such a fuel cell, hydrogen gas supplied through a gas passage facing on a gas diffusion electrode of the anode and oxidizing gas such as air or oxygen supplied through a gas passage facing on a gas diffusion electrode of the cathode electrochemically react and thereby generate electricity.
The separator supplies electrons generated by catalytic reaction of hydrogen gas at the anode side to an outer circuit, and on the other hand, the separator must have a function of supplying electrons from the outer circuit to the cathode. Therefore, a conductive material comprising, for example, a graphite-based material or a metal-based material, is used as the separator. In particular, the metal-based material is advantageous since it is superior in mechanical strength, and the weight and size can be reduced by reducing the thickness of the metallic plate. A metallic separator in which a thin plate of stainless steel or titanium alloy having high corrosion resistance is pressed to have uneven cross section can be used.
As the environmental factors when such a separator is operating in a fuel cell, following three factors can be considered.
1. Temperature: Since an operation temperature of a fuel cell is in a range from ordinary temperature to about 180° C., the separator is also used in this range.
2. pH: In a fuel cell, oxygen and hydrogen react to generate water. The water is generated as a vapor, although water droplets may form generated on the separator if the temperature of water vapor in the gas passage decreases. If the amount of water in droplets increases, water remains between the membrane electrode assembly and the separator. The water which adheres to the membrane electrode assembly is easily brought into contact with the electrolyte membrane. Therefore, substituents of the electrolyte membrane are liberated, hydrogen ions are generated in the water, and pH of the water is decreased. As a substituent of a fuel cell, sulfonic groups are common. Therefore, the water becomes acidic, and is, for example, such as sulfuric acid.
The substituent mentioned above is further explained. In a fuel cell, hydrogen ions generated from hydrogen at the catalyst at the hydrogen gas supplying side (anode) are supplied to a cathode catalyst. Then, the hydrogen ions react with oxidizing gas in the cathode catalyst, and thereby generate electricity continuously. Therefore, an electrolyte membrane of a fuel cell must be an electrolyte membrane of the cation conductive type in which hydrogen ions move from the anode side to the cathode side. Therefore, a functional group which combines with hydrogen ions must be present in a side-chain of the electrolyte membrane molecule. In a fuel cell, an acidic type substituent which combines with hydrogen ions is arranged in part of molecules of the electrolyte membrane to perform the above-mentioned function. Since this substituent is of the acidic type, acid is generated if the substituent is liberated from the electrolyte membrane. Generally, to increase transport efficiency of hydrogen ions in electrolyte membranes, a strongly acidic type which strongly combines with hydrogen ions is used as this substituent. Therefore, the substituent is liberated, generating acid, and this acid reduces the pH.
3. Electric potential: The separators are arranged at the fuel gas side and the oxidizing gas side and work as the cathode and the anode of the cell. Electromotive force by the reaction occurs between these two separators as a potential difference. Generally, the maximum potential difference occurring by the electromotive force of the fuel cell in which hydrogen as a fuel gas and oxygen as an oxidizing gas are used, is about 1.2 V for the following reason. That is, in the range of operating temperatures of a fuel cell, electromotive force obtained by the chemical reaction in which water is generated by hydrogen and oxygen is about 1.2 V by theoretical calculation. In actual power generation, about 1 to 1.2 V can be generated. In the case in which an austenite based stainless steel plate having high corrosion resistance is used as the separator, the elution rate of metal ion is increased at over 0.9 V of electromotive force, and the separator may be corroded.
Since the separator of a fuel cell may be easily corroded under the conditions of temperature, pH, and electric potential as explained above, even in the case in which a metallic separator having high corrosion resistance (such as SUS316L) is used, the separator may be easily corroded. Therefore, extremely high corrosion resistance is required for the metallic separators in the operating environments of fuel cells. Furthermore, it is necessary for the metallic separator that a pressing process in which an uneven cross section is formed can be easily performed to form gas passages or coolant passages, and that contact resistance with other members be extremely low to prevent the generated voltage from deteriorating. Furthermore, low cost is also required because hundreds of separators may be used in one fuel cell stack in certain situations.
Therefore, as a separator for fuel cells, it is said to be desirable that a metal having high corrosion resistance be plated on a surface of a stainless steel which can be easily pressed. Corrosion resistance of SUS316L (stainless steel), Cu, Ag, Pt, and Au was compared by measuring each corrosion current density under conditions of temperature 90° C., voltage 1.2 V, in sulfuric acid solution of pH 3. The results were as follows.
SUS316L: 156 μA/cm2 
Cu: 98 μA/cm2 
Ag: 38 μA/cm2
Pt: 18 μA/cm2 
Au: 2 μA/cm2 
It is desirable that the corrosion current density be not more than 10 μA/cm2 to maintain durability of the fuel cell in practical use. It is obvious that the metal which fulfills this requirement is gold. Therefore, a material such as a stainless steel plate which is plated with gold is desirable as a separator for a fuel cell.
However, in the case in which gold is plated on the stainless steel by an ordinary method, adhesion is not very high since gold merely adheres on stainless steel physically. Therefore, in the case in which a separator whose cross section is uneven and whose radius of curvature of a part is extremely small is formed by a pressing process, plated gold may be easily exfoliated. Furthermore, during prolonged operation of a fuel cell, gold may be lost since gold is a solid-solute and diffuses into Fe, Cr, and Ni which are main components of stainless steel. Therefore, high corrosion resistance and low contact resistance cannot be exhibited.